Boolean Modifier (Difference) Issue Persists Even After Applying All Transformations, Recalcing Normals, and Fixing Non-Manifolds
There are were inverted normals, so Shift+N to recalculate them. 
Overlapping edges between boolean cutter and source object. This is healed by moving edge away from the cut. It's a front view: yellow edge is source, white is cutter:


Lots of ngons, so consider doing retopology. I added Triangulate modifier to make it more boolean-friendly:




Also keep boolean object's geometry as simple as posible, avoid excessive edges which doesn't contribute to final shape.